Uninterruptible power supplies contain a battery or other power source that enables the power supply to continue to supply energy after main power has been lost. Uninterruptible power supplies are frequently used to supply power to electronic equipment such communications equipment or computers that operate in an environment wherein a sudden loss of power is not acceptable. Depending upon the power consumption of the operating equipment and the capacity of the auxiliary power source, the auxiliary power source may be called upon to supply auxiliary power for only a few minutes while data is saved and an orderly shutdown is executed or for the duration of a power failure, that could last for hours or even days.
Technological improvements that have been achieved in recent years have enabled uninterruptible power supplies to be manufactured with increased reliability and reduced cost. This has increased the number of applications in which uninterruptible power supplies can be economically employed, thereby further increasing the demand for such power supplies. This increased demand makes further improvements in reliability and efficiency as well as reductions in cost even more important.
For safety reasons, power supplies are required to isolate the high voltage power input side of a power supply from the lower voltage output side. The auxiliary power supply is typically a low voltage battery of about 12 to 24 volts and therefor is usually connected to the low voltage side of the power supply. The battery must be isolated from the secondary windings to prevent overcharging the battery and to prevent drawing power from the battery when main power is present. In one arrangement shown in U.S. Pat. No. 5,289,045 to Lavin et al., isolation is achieved by enabling the power switch for auxiliary current to operate only when a loss of main power is detected. Extra circuitry is thus required to detect the power loss and selectively enable power switch operation.
Other arrangements place an isolation diode between the auxiliary battery and the secondary winding. As long as main power is present the diode remains reverse biased and the battery is protected. When main power is lost the voltage at the secondary winding drops below the battery voltage and the battery begins to supply current to the secondary winding through the diode. While the diode isolation avoids the need for power loss detection circuitry, it has certain disadvantages. The diode must carry a large current with a minimum voltage drop and is relatively expensive. The 0.6 volt drop across a diode represents a significant 5% power efficiency loss for a 12 volt battery. This power loss not only reduces the auxiliary backup time in the event of a loss of main power, but also increases the heat dissipation load on both the power supply and the equipment in which the power supply is installed. A relatively expensive, high current voltage regulator is required to charge and maintain the battery regardless of which type of isolation is used.
A need thus exists to improve both the cost and efficiency of uninterruptible power supplies. By regulating the voltage on the secondary winding in accordance with the present invention, the need for both the battery isolation circuit component and the separate battery charging circuit is eliminated.